Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased. Among these secondary batteries, lithium secondary batteries having high energy density and high voltage have been commercialized and widely used.
A lithium metal oxide is used as a positive electrode active material of a lithium secondary battery, and lithium metal, a lithium alloy, crystalline or amorphous carbon, or a carbon composite is used as a negative electrode active material. A current collector may be coated with a positive electrode active material or negative electrode active material of appropriate thickness and length or the active material itself may be coated in the form of a film, and the resultant product is then wound or stacked with an insulating separator to prepare an electrode assembly. Thereafter, the electrode assembly is put into a can or a container similar thereto, and a secondary battery is then prepared by injecting an electrolyte solution.
An electrolyte in a liquid state, particularly, an ion conductive liquid electrolyte, in which a salt is dissolved in a non-aqueous organic solvent, has been mainly used as an electrolyte for an electrochemical device, such as a typical battery using an electrochemical reaction and an electric double-layer capacitor.
However, when the electrolyte in a liquid state is used, an electrode material may degrade and the organic solvent is likely to be volatilized. Also, there may be limitations in safety such as combustion due to increases in ambient temperature and the temperature of the battery itself. In particular, a lithium secondary battery has limitations in that gas may be generated in the battery due to the decomposition of a carbonate organic solvent and/or a side reaction between the organic solvent and an electrode during charge and discharge to expand the thickness of the battery, and such reactions are accelerated during high-temperature storage to further increase an amount of the gas generated.
The continuously generated gas causes an increase in the internal pressure of the battery to not only result in decreased safety, for example, a prismatic type battery is swollen in a specific direction and exploded, or the center of a specific surface of the battery is deformed, but also reduces the performance of the battery by generating local differences in adhesion on the surface of the electrode in the battery to prevent the electrode reaction from occurring across the entire surface of the electrode in the same manner.
As interests in energy storage technologies have been increasingly grown, three is a need to develop a secondary battery capable of being miniaturized and lightweight as well as being charged and discharged with high capacity. Accordingly, development of a battery using a polymer electrolyte formed of a polymer, instead of using a liquid electrolyte, has recently received attention.
In general, it is known that battery safety improves in the order of a liquid electrolyte, a gel polymer electrolyte, and a solid polymer electrolyte, but battery performance decreases in the same order.
That is, the gel polymer electrolyte may have lower lithium ion conductivity than a liquid electrolyte formed only of an electrolyte solution. Thus, a method of decreasing the thickness of the gel polymer electrolyte has been proposed in order to improve the conductivity. However, in this case, there may be limitations in improving the performance and safety of the battery, for example, mechanical strength is decreased and a short-circuit of the polymer electrolyte occurs due to a short-circuit of a positive electrode and a negative electrode during the preparation of the battery.
Therefore, there is a need to develop a gel polymer electrolyte having improved performance and safety of the battery.